1. Field of the Invention
The present invention relates to a chip stack package and more particularly to a semiconductor chip, a chip stack package, and method for manufacturing.
2. Description of Related Art
In an atmosphere of incessant pressure to reduce the size of electronic products while maintaining and increasing their functionality, extreme attention is given to every single element of an electronic assembly. Because of growing pressure to reduce size, a whole new class of IC package, especially flip chip technique and chip-scale package techniques were introduced. However, as good as these devices are at reducing the size of electronics assemblies, they too have been unable to placate the appetite of the consumer for increasing functionality of electronic products.
As a result, product engineers and designers have turned their attention to the third dimension in IC packaging. Stack techniques have emerged as an innovative means to meet the market requirements of next generation electronic products. There are variations to the stack technique such as stacking semiconductor chips or stacking packages.
The later stack package has functional diversity, since various packages are assembled, and improved reliability since the assembled elements passed functional and reliability tests. Further, this stacked package has an advantage of increased memory capacity. Since two or more packages are stacked, the size of the package increases and therefore it is very difficult to handle.
On the other hand, to solve the foregoing problem the former chip stack package has at least two semiconductor chips in a package. This stacked package has an advantage of memory capacity and functionality that can be efficiently increased. Further, the thickness of the stack device can be further reduced when compared with the above package stack device. However the larger and larger chip stacks contain multiple chips which incur cooling problems. Because the chip stack contains multiple chips, they generate more heat per unit volume, requiring greater thermal dissipation. Moreover, this dissipation must occur now through layers of semiconductor substrate material.
FIG. 1 is a sectional view showing a conventional chip stack package available from Vertical Circuit Inc., U.S.A. As shown in FIG. 1, the chip stack package 100 has a plurality of semiconductor chips 110. The semiconductor chips 110 have an active surface 112 formed with bonding pads 111 and a backside 113 that is at the rear of the active surface 112. An insulation layer 114 covers the entire surface of the semiconductor chips 110 only exposing bonding pads 111 for conductive bumps 115. The conductive bumps 115 are attached between the bonding pads 111 of lower chips 110b, 110c, 110d, 110e, 110f and 110g and the backside 113 of respective upper semiconductor chips 110a, 110b, 110c, 110c, 110d, 110e and 110f. 
A heat sink 150 is placed between each two neighboring semiconductor chips 110 for ensuring efficient heat emission from each semiconductor chip 110. The lowest semiconductor chip 110g is attached on a silicon substrate 120. The stacked chips 110 and substrate 120 are electrically interconnected by the conductive bumps 115 and a conductive adhesive 160. Lastly, for electrical connection of the chips 110 externally, the silicon substrate 120 is bonded to lead frame 130 using conventional die-attach and wire-bonding processes. Then the structure 100 is molded, formed and trimmed.
The signal path in the chip stack package 100 is from an external device to the lead frame 130, the bonding wires 140, the silicon substrate 120, the conductive adhesive 160, each conductive bump 115, and to each semiconductor chip 110a to 110g. During operation of the chips 110, heat is generated, which is emitted to the outside through the heat sink 150.
In this chip stack package 100, the interconnection of each semiconductor chip 110 and silicon substrate 120 is made by the conductive adhesive 160 instead of wires conventionally used in packaging technology. Many problems occurring due to using the wires, for example, wire sweeping at higher wire loop in molding process, increased signal path length and increased inductance are resolved by using the conductive adhesive 160. The chip stack package 100 can gain higher speed and lower noise levels by reducing the signal inductance and power/ground inductance.
However, when the chip stack package 100 includes different types of semiconductor chips 110, the conductive adhesive 160 is not easily formed due to different dimensions of the chips 110 in profile. This causes difficulty in realizing a multi-functional stack package by using different chips. Additionally, it is hard to efficiently inject the conductive adhesive 160 into a space between neighboring semiconductor chips 110, which results in poor electrical connection with the conductive bumps 115. Therefore, the conventional chip stack package 100 cannot employ a center pad type chip in which the bonding pads are centrally disposed on the active surface, but an edge pad type chip in which the bonding pads are peripherally disposed on the active surface.